Professor Lesley Gray

Abstract:

Observational evidence of the 11-year solar cycle (SC) modulation of stratosphere temperatures and winds from the ERA-40 dataset is reviewed, with emphasis on the Northern winter hemisphere. A frequency modulation of sudden warming events is noted, with warmings occurring earlier in solar minimum periods than in solar maximum periods. The observed interaction between the influence of the SC and the quasi biennial oscillation (QBO) on the frequency of sudden warmings is noted as a possible clue for understanding their mechanism of influence. A possible transfer route for the 11-year solar cycle from the equatorial stratopause region to the lowest part of the stratosphere is proposed, via an influence on sudden warming events and the associated induced meridional circulation. SC and QBO composites of zonal wind anomalies show anomalous wind distributions in the subtropical upper stratosphere in early winter. Mechanistic model experiments are reviewed that demonstrate a sensitivity of sudden warmings to small wind anomalies in this region. Various diagnostics from these experiments are shown, including EP fluxes and their divergence and also the synoptic evolution of the polar vortex, in order to understand the mechanism of the influence. Some recent GCM experiments to investigate the SC/QBO interaction are also described. They simulate reasonably well the observed SC/QBO interaction of sudden warming events and appear to support the hypothesis that tropical/subtropical upper stratospheric wind anomalies are an important influence on the timing of sudden warmings. © Springer Science+Business Media, Inc. 2007.

Abstract:

A full troposphere-stratosphere-mesosphere global circulation model is used in a set of idealised experiments to investigate the sensitivity of the Northern Hemisphere winter stratospheric flow to improvements in the equatorial zonal winds. The model shows significant sensitivity to variability in the upper equatorial stratosphere, the imposition of SAO and QBO like variability in this region advances the timing of midwinter sudden warmings by about one month. Perturbations to the lower equatorial stratosphere are mainly found to influence early winter polar variability. These results suggest that it is important to pay attention to the capability of models to simulate realistic variability in the upper equatorial stratosphere. Copyright 2006 by the American Geophysical Union.

Abstract:

The impact of solar irradiance changes on the winter polar stratosphere is investigated using a general circulation model in which the equatorial Quasi-Biennial Oscillation (QBO) is internally generated and self-sustaining. The model results compare favorably with observations, supporting previous findings that the equatorial zonal wind modulates the polar stratospheric response to solar irradiance changes. In the QBO easterly phase, Northern Hemisphere sudden stratospheric warmings are found to be more prevalent under solar minimum conditions than under solar maximum conditions. However, in the QBO westerly phase the reverse is true. The possible solar-modulation of the QBO period is also investigated. Although small changes are evident in the same sense as those observed, i.e. a lengthening of the period during solar minimum conditions, longer simulations would be required to verify the statistical significance of this result. Copyright 2005 by the American Geophysical Union.

Abstract:

ECMWF ERA-40 reanalysis zonal winds are very close to tropical rocketsonde and rawinsonde (radiosonde & radar wind soundings) observations up to 10 hPa. Above 10 hPa differences increase, although the ERA-40 data provide a good representation of tropical winds up to 2-3 hPa. The amplitudes of the quasi-biennial oscillation (QBO) and the semi-annual oscillation (SAO) derived from ERA-40 data match the rawinsonde and rocketsonde observations up to 2-3 hPa. We conclude that zonal-mean ERA-40 equatorial winds could be used, for most purposes, in place of rawinsonde station observations. Copyright 2005 by the American Geophysical Union.